Modern inorganic chemistry

132 GROUPS) AND I!

THE CARBONATES AND HYDROGENCARBONATES

As with the hydroxides, we find that whilst the carbonates of most

metals are insoluble, those of alkali metals are soluble, so that they

provide a good source of the carbonate ion COf ~ in solution; the

alkali metal carbonates, except that of lithium, are stable to heat.

Group II carbonates are generally insoluble in water and less

stable to heat, losing carbon dioxide reversibly at high temperatures.

Table 6.4

DECOMPOSITION TEMPERATURES* (K) OF SOME CARBONATES

Group 1 Group II

Li 2 C0 3

Na 2 CO 3

K 2 C0 3

Rb 2 C0 3

Cs 2 CO 3

1540

v. high

v. high

v. high

v. high

BeCO 3

MgC0 3

CaCO 3

SrC0 3

BaC0 3

370

470

1 170

1550

1630

* The temperature at which the pressure of CO 2 reaches 1 atmosphere.

A further peculiarity of the Group I and II carbonates is the ability

to form the hydrogencarbonate or bicarbonate ion HCOa:

CO?" + H 3 O+ ^ HCOJ + H 2 O

This ion is produced by the prolonged passage of carbon dioxide
through neutral or alkaline solutions containing Group I or II ions
(except lithium or beryllium which do not form a hydrogencarbon-
ate). The hydrogencarbonates of Group 1 elements can be isolated
as solids but these solids readily decompose when heated to form
the carbonate with the evolution of carbon dioxide and water, for
example

2NaHCO 3 -> Na 2 CO 3 + H 2 O + CO 2

Group II hydrogencarbonates have insufficient thermal stability for
them to be isolated as solids. However, in areas where natural
deposits of calcium and magnesium carbonates are found a reaction
between the carbonate, water and carbon dioxide occurs:

M"CO 3 + CO 2 + H 2 O -> M^2 + + 2H(X>3

Insoluble In solution

This produces sufficient concentrations of magnesium and calcium
ions to render the water hard. The above reaction is readily reversed
by boiling the water when the magnesium and calcium ions res-
ponsible for the hardness are removed as the insoluble carbonate.